Erosion is the detachment and movement of soil particles or geologic material through naturally-occurring processes such as wind, rain, and running water occurring either as overland flows or as concentrated flows. Surface water runoff is the primary cause of erosion. Overland flow occurs when the rainfall rate exceeds the rate of infiltration and evaporation for a particular kind of soil, once the detention storage capacity of the soil becomes filled. Topographical features such as slope length and slope gradient also influence erosion. Soil texture, classification and compaction, and ground cover such as vegetation and rocks are other influencing factors.
Erosion occurs when the protective vegetative cover and organic layer are removed from mineral soils. Although raindrops and splash erosion are concerns relating to dislodging and transporting soil particles, this invention primarily deals with reducing sheet, rill and gully erosion forces and inducing the deposition of sediment entrained in concentrated flows of water. Erosion takes place when surface water runoff, the transporting agent for particles of soil, has enough energy to entrain and move sediment. This energy is a function of the amount and velocity of water passing over a given area in a given time. Accelerated erosion rates cause rills and gullys, indicating the erosive forces are overcoming the slope resistance.
Sheet erosion occurs when water flows along the surface of a slope with energy sufficient to entrain said on the slope, without cutting a particular channel into the surface. Rill erosion occurs once there is concentrated flow with enough energy to incise the surface of a slope. At this stage, the shear force of the flow exceeds the shear resistance of the surface. Gullys become formed downslope of rills, often through the interception of more than one rill. The additive flow from several rills increases the erosive energy, causing further incision of the surface. Rills over one foot deep are generally considered gullys. Gullys are a significant form of direct erosion by surface water runoff, and it is desirable to prevent gullys by controlling the growth of rills.
Prior art attempts to control soil erosion arising from surface water runoff generally have relied on mats or blankets made from natural or manmade fibers being placed on the ground surface to protect the surface from the erosive effect of wind or water movement over the surface. Slope control, while vegetation takes hold on the surface, has been the principal application of such mats. Typical mats for the purpose are flexible or semiflexible sheets manufactured to predetemined dimensions, and provided with a scrim or other mechanical structure providing a framework for defining and maintaining the dimensional stability of the structure. U.S. Pat. No. 4,610,568 entitled "Slope Stabilization System and Method" discloses a fabric layered mat of that general kind. These manufactured structures typically must be cut in the field to fit a particular surface of application, a procedure which is time-consuming and awkward as the natural slope and shape of the soil seldom conforms to the relatively linear shapes of such manufactured products. This problem is particularly evident when attempting to fit existing erosion-control mats into the existing irregular cross-sectional shapes of rills or gullys.
Other attempts to control erosive flow through rills and gullys include emplacing bales of hay or straw within the gully. The overall external shapes of such bales are effectively fixed during the baling process, and those shapes thus are not readily conformable to the irregular areas of gullys or rills. Hay bales present the further disadvantage of acting like a dam to block the water flow. The water will go around or over the bale when the rill or gully becomes flooded. The velocity of this water is increased in the bypass region and therefore has a much greater erosive force. Moreover, hay or straw and the twine used to bind the bales usually biodegrade at so rapid a rate that the structural integrity of the bales is lost before the rill or gully becomes effectively blocked by the process of sedimentation.
Other methods of erosion control include the use of woven, generally open-cell natural or manmade fiber yarns which protect the slope, vegetation and plants early in their growth process. Scattering bark or mulch and the use of spray mulch are additional devices for temporary slope control. These methods are used early in the construction or reconstruction process for steep to moderate slopes. Once in place, the fabrics or other materials biodegrade or photodegrade. Should there be a failure of the vegetative or armoring process, channels form with progressing impact on the erosive action phenomenon.